1. Field of the Invention
The present invention relates to a magnetic recording medium and more particularly to a magnetic recording medium, particularly based on an in-plane magnetization recording system, which, through a reduction in thickness of the magnetic recording layer, can realize higher recording density and, despite the reduction in thickness of the magnetic recording layer, is thermally stable and can exhibit a satisfactorily high coercive force. Further, the present invention relates to a process for producing the magnetic recording medium, and a disk, for recording and reproducing information, using the magnetic recording medium.
2. Description of the Related Art
The development of information processing techniques has led to an increasing demand for an increase in recording density of magnetic disk devices used for external storage in computers. Specifically, in the reproducing head of the magnetic disk devices, the use of a magnetoresistive head utilizing a magnetoresistor, wherein the electric resistance changes in response to the magnetic field intensity, that is, an MR head, instead of the conventional wound-type inductive thin film magnetic head, has been proposed in the art. The MR head has applied magnetoresistance, that is, the change in electric resistance produced in a magnetic material on application of an external magnetic field, to the reproduction of a signal on a recording medium and has features including that the reproduction output margin is several times larger than that in the case of the conventional inductive thin film magnetic head, the inductance is small and a large S/N ratio can be expected. Further, the use of an AMR (anisotropic magnetoresistive) head utilizing anisotropic magnetoresistance, a GMR (giant magnetoresistive) head utilizing giant magnetoresistance, and a spin valve GMR head of practical type, besides the MR head, have also been proposed.
Further, in order to meet the demand for high-density recording, a sufficient improvement in properties to cope with the above MR head, AMR head, or GMR head (including spin valve head) has been demanded also in the magnetic recording medium used in the magnetic disk device. In particular, low tBr (a product of the thickness t and the residual magnetization density Br of a magnetic recording layer), low noise level, and high coercive force Hc are required of the magnetic recording medium. In order to meet such requirements, the prior art, for example, Japanese Unexamined Patent Publication (Kokai) No. 1-256017, discloses a magnetic recording medium comprising a nonmagnetic substrate, a chromium layer (thickness=100 nm) as an underlayer provided on the nonmagnetic substrate, and a magnetic layer (thickness=60 nm) provided on the chromium layer, the magnetic layer being formed of a CoCrTaPt-base four-component alloy and having a combination of a low noise level derived from CoCrTa-base alloy with high coercive force derived from CoCrPt-base alloy. U.S. Pat. No. 5,004,652 and its Japanese counterpart, Japanese Unexamined Patent Publication (Kokai) No. 4-228105 discloses a magnetic recording medium comprising a nonmagnetic substrate and a chromium layer (thickness=about 30-300 nm) and a magnetic layer of a CoCrPtTa four-component alloy (thickness=about 20-100 nm) formed in that order by sputtering on the nonmagnetic substrate. Further, Japanese Unexamined Patent Publication (Kokai) No. 5-72016 discloses a process for producing a magnetic recording medium, wherein a magnetic metallic thin layer is sputtered on a substrate.
Further, Japanese Unexamined Patent Publication (Kokai) No. 6-76279 discloses a thin-layer metal alloy magnetic recording disk comprising: a substrate, containing an aluminum alloy, having thereon a NiP surface covering; an NiO layer provided on the NiP covering; an underlayer provided on the NiO layer; and a magnetic layer, provided so as to cover the underlayer, comprising an alloy composed mainly of cobalt. In this magnetic recording disk, the NiO layer has been formed by oxidation of the NiP covering, offering advantages including that the smoothness of the NiP covering is retained as it is, offering a smoother disk surface, scratches finally left in the step of polishing the surface of the NiP covering can be eliminated, and the magnetic layer, which is later deposited by sputtering on the NiO layer, can have high saturated coercive force, enabling the disk to be used in a disk file system in which a recording head is contacted with the disk.
In the above unexamined patent publications and other patent publications, the recording density and the coercive force of the magnetic recording medium are simultaneously enhanced particularly through an improvement in composition of the magnetic recording layer. However, an improvement in thermal stability, which, in the present invention, is expected to be attained simultaneously with the enhanced recording density and the coercive force, remains unattained. Specifically, a reduction in the thickness of the magnetic recording layer in the magnetic recording medium results in improved resolution, realizing recording at higher density. When the layer thickness is reduced beyond a limit, the improved coercive force, on the contrary, is lowered and, in addition, the thermal stability becomes poor, making it impossible to use the material as a magnetic recording medium. In particular, the problem of thermal instability in the magnetic recording medium occurs at room temperature, and, hence, complete solution to this problem has been desired in the art.